In recent years, much effort has been devoted to map the inside of diffusive bodies using ultrasound and electromagnetic waves. If a sound (or ultrasound) wave is located inside a scattering medium and concurrently a continuous electromagnetic wave (such as a laser light beam) crosses said medium and is strongly diffused thereby, the electromagnetic wave frequency is shifted by the sound frequency (acousto-optic effect) at the location of the sound (or ultrasound) wave, while in the other regions, the frequency of the light is unchanged. The electromagnetic waves with the shifted or tagged frequencies are detected. Since the location of the ultrasound waves inside the medium, and consequently, the locations of interaction between the ultrasound and electromagnetic waves can easily be determined, a 3-D representation of the medium can be obtained.
Lev A. et al. “Ultrasound probing of the banana photon distribution in turbid media”, Biomedical Optoacoustics II, San Jose, Calif., USA, 23-24 Jan. 2001, vol. 4256, pp. 233-240 discloses the possibility of ultrasound tagging of light to map the photon density inside solid turbine media. The modulation of the optical field transmitted through a scattering medium by an ultrasound beam is also disclosed in M. Kampe et al. “Acousto-optic tomography with multiply scattered light”, Optical Society of America, 1997, pp. 1151-1158.
It has been shown [Optics Letters, Lev et al, March 2000] that the technique of ultrasound tagging of light provides for locating an electromagnetic wave absorbing object within a non-absorbing, diffusive medium. However, it appears that when the are several absorbing objects or the single object has a pattern of absorbing locations within the diffusive medium, a correlation between the absorption of the different objects/locations occurs, and the so-obtained 3-D representation is insufficient to provide an exact picture of the absorbing pattern within the medium, and data reconstruction is thus required.
Image reconstruction techniques typically used with optical measurements utilize inverse scattering algorithms [S. R. Arridge and J. C. Hebden, “Optical imaging in medicine II. Modeling and reconstruction”, Physics in Medicine and Biology 42, 841-853 (1997)]. In these methods light scattered from the medium is detected enabling a two dimensional data representation. This two-dimensional data is then reconstructed into a three-dimensional pattern of absorption (or scattering). The results of such techniques are limited by several factors: the optical measurement methods are very sensitive to boundary conditions (sensors or sources positions), the data reconstruction requires long computation time, and the image resolution is relatively low, e.g., general not exceeding 5 mm in the case of optical tomography.